1. Field of the Invention
The present invention relates to a fuel cell separator assembly having a metal diffusion layer and a metal separator, relates a manufacturing method therefor, relates to a fuel cell unit including the separator assembly, and relates to a fuel cell stack including the fuel cell units.
Priority is claimed on Japanese Patent Application No. 2002-303043, filed Oct. 17, 2002, and Japanese Patent Application No. 2002-303045, filed Oct. 17, 2002, the contents of which are incorporated herein by reference.
2. Description of Related Art
In the field of fuel cells, a fuel cell unit is known which includes, for example, a solid polymer electrolyte membrane, an anode electrode and a cathode electrode that together sandwich the solid polymer electrolyte membrane, metal diffusion layers respectively disposed outside of the electrodes, and metal separators respectively disposed outside of the metal diffusion layers. In practice, such fuel cell units are stacked together to form a fuel cell stack.
In general, because a diffusion layer has low mechanical strength, it is difficult to handle the diffusion layer. Moreover, when fuel cell units are assembled, or when a fuel cell stack is assembled while handling the diffusion layers and the separators as independent elements, many elements must be handled, which may lead to poor assembling efficiency, and may lead to too many items being managed, which is troublesome.
In order to solve the above problems, a unified separator assembly has been developed in which a diffusion layer and a separator are connected and unified. For example, a diffusion layer and a separator may be unified using an adhesive or by using clips.
In the subsequent assembly process, a fuel cell stack may be formed by laser-welding an end of an anode side separator and an end of a cathode side separator which are disposed at two sides of a solid polymer electrolyte membrane (see, for example, Japanese Unexamined Patent Application, First Publication No. Hei 08-255616).
In a fuel cell stack including the above-mentioned conventional separators, the separators are configured by press-forming metal plates so that concave and convex portions are formed therein, and grooves which are formed between the separators and diffusion layers are used as fuel flow passages and oxidizer flow passages. Moreover, grooves which are formed between the fuel cell units adjacent to each other are used as coolant flow passages.
In the case in which the diffusion layers and the separators are unified using an adhesive, productivity is inferior because a number of manufacturing processes are required, including a coating process for an adhesive and a curing process for the adhesive, and because, in the curing process, the adhesive must be cured for a several hours at high temperature or at normal temperature.
On the other hand, in the case in which the diffusion layers and the separators are unified by holding them using clips, most of the resistance overvoltage during power generation is occupied by the contact resistance between the diffusion layers and the separators, and the contact pressure between the diffusion layers and the separators must be increased in order to reduce the resistance overvoltage. However, in order to increase the contact pressure, the fuel cell units and the fuel cell stack must have a relatively high rigidity, which leads to increase in size and weight of the fuel cell units and the fuel cell stack.
Furthermore, as in the case of conventional metal separators in which flow passage partitions are configured by press-forming, flow passages cannot be freely configured due to manufacturing limitations in the press-forming process.
In this case, the flow passages are formed on both sides of each of the separators in such a manner that the flow passage in one surface (the fuel flow passage or oxidizer flow passage) and the flow passage in the other surface (the coolant flow passage) are disposed in parallel with and adjacent to each other; therefore, an optimum flow passage for one fluid cannot be formed without being affected by another flow passage for another fluid.
Moreover, because the separators have the concave and convex portions, the thickness of each of the separators in the stacking direction is large, which leads to increase in thickness of the fuel cell units and the fuel cell stack in the stacking direction, i.e., leads to an increase in size of the fuel cell stack.
In addition, if the configuration of the flow passages must be changed, new press molds must be prepared for the new configuration of the flow passages; therefore, change of the configuration of the flow passages is not only difficult but is also expensive.